This invention pertains to a device for igniting fuel injected into a rapidly flowing gaseous comburent medium. It is particularly applicable to ignition of reheaters (or afterburners) of aircraft turbojet engines, but it may also be applied to the ignition of ramjet engines, for example.
Aircraft turbojet reheaters are normally ignited by means of an injection of fuel during a limited interval, upstream of at least the last stages of the turbojet turbine. Since the pressure and temperature are very high at this point, the fuel ignites spontaneously and produces a local overheating of the gases, leading to ignition of the carbureted reheating mixture. This mode of ignition, known in the art by the English expression "hot streak", involves risks, since the failure of an injection valve or rupture of a feed line results in a prolonged discharge of fuel through the turbine, leading to its destruction.
Moreover, when the reheater is used during take-off, it is desirable to be able to ignite it before the aircraft starts to roll on the ground, so as to ensure that it will be ignited upon take-off. To this end, one must be able to ignite the reheater while the turbojet engine is operating at partial load, because the aircraft's brakes are incapable of holding it fast while the turbojet is operating at full power and the afterburner is also on. Since it is impossible to ignite the reheater of a turbojet engine operating at partial load by means of the "hot streak" method, it has been proposed to inject ignition fuel into a small auxiliary chamber, known as the ignition "prechamber", so as to form within it a carbureted mixture of a richness suitable to ensure that it will be ignited by an ignition element such as a spark plug. As the auxiliary chamber ejects the burning gases near a flame stabilizer located in the after burner channel downstream of an injection of reheating fuel, into a region where the richness of the carbureted mixture is sufficient, the combustion propagates through the entire channel.
In such "prechamber"-equipped devices, it has been sougt to obtain an intimate mixture of the ignition fuel with the comburent air by directing the fuel onto a grid or perforated plate. Such an arrngement does not permit igniting the reheater under a very fast flow of gas. It has been observed, for example, that ignition does not take place if the Mach number of the gas in the afterburner channel reaches 0.40 at an "afterburner overall richness" (ratio of the weight rate of flow of the fuel injected into the afterburner channel to the weight rate of flow of the air contained in the gas current flowing in that channel) of 0.003 to 0.004. Examples of prior devices seeking to achieve more reliable afterburner ignition are the devices described in U.S. Pat. No. 2,920,445 and French Pat. No. 1,207,017.